The third Gordon Research Conference on Motile and Contractile Systems will provide an informal forum for current advances and directions in this rapidly developing field. The focus of the meeting will be the dynamic nature of the actin and microtubule filament systems, their activation signals and their motors. These topics are relevant to many disciplines including musculoskeletal disease, development, cell transformation and oncogenesis, as well as normal functioning of the organism. The conference will include approximately 30 formal presentations by invited speakers with major emphasis on discussion by all participants. Junior scientists especially, will be encouraged to present posters of their work and one entire session will be devoted to poster discussion and short presentations. This format has worked well for the past two conferences, and we plan to continue emphasizing a minimum of formal presentations to maximize both spontaneous exchange and the opportunity to introduce significant new observations. The conference will begin with a session (#1) on microtubule dynamics, presenting evidence for dynamic instability as well as microtubule treadmilling. This will be followed by a session (#2) on membrane and cytoskeletal dynamics, which illustrates the rapidity with which actin filaments move, and suggests a linkage between membrane and cytoskeletal movement. The next session (#3) will highlight new discoveries about nonconventional myosins, particularly their membrane- association. This extends that list of membrane-associated actin binding proteins, and leads into the next topic of the role of membrane cytoskeleton in signal transduction (#4). We will next shift to domain analysis of some of the actin regulatory proteins, with particular emphasis on methods to identify and deduce important functional domains (#5). The biology of microtubule based motors which generate retrograde and anterograde movement of organelles along microtubules will be presented in session #6. Session #7 introduces exciting developments in the field of muscle determination and differentiation, which should be relevant to all interested in contractility and motility. Session #8 will focus on recent advances using molecular genetic approaches to probe the in vivo functions of cytoskeletal proteins. Session #9 will be used for a review and discussion of the most noteworthy posters. An informal film session for presentation of video tape recordings of cell movement will be included one afternoon.